This refers to the pic(1) preprocessor to the troff(1) typesetter. Troff is a historical typesetting software with a long history in the Unix ecosystem. Having personally used some of the modern implementations (e.g. GNU troff or groff, heirloom troff) of troff for personal projects, it has been a pleasant experience.
https://troff.org
https://en.wikipedia.org/wiki/Troff
https://en.wikipedia.org/wiki/PIC_(markup_language)
https://www.man7.org/linux/man-pages/man7/roff.7.html